The present disclosure relates to solid-state image sensors and manufacturing methods of the sensors, and more particularly to solid-state image sensors using silicon-on-insulator (SOI) substrates and manufacturing methods of the sensors.
Solid-state image sensors such as charge coupled devices (CCDs), CMOS image sensors, etc., are generally used for digital cameras, video cameras, etc. In recent years, with an improvement in solid-state image sensors, high-definition images can be obtained by a solid-state image sensor with highly dense pixels.
A conventional solid-state image sensor includes a transfer gate, a photoelectric conversion element, a MOS transistor, various interconnects, etc. on a semiconductor substrate. However, a light-receiving region of a photoelectric conversion element needs to be provided avoiding the transfer gate, the MOS transistor, the interconnects, etc. There is thus the problem that the aperture ratio of the light-receiving region reduces with a reduced pixel size due to reduction in the area of the semiconductor substrate and an increase in the number of pixels.
Thus, in recent years, increasing attention has been given to a solid-state image sensor of back surface irradiation including a transfer gate, a MOS transistors, and an interconnect layer on a first (front) surface of a semiconductor substrate, and a photoelectric conversion element on a second (back) surface so that the back surface serves as a light-receiving region.
A substrate of a solid-state image sensor of back surface irradiation needs to be thinned to a thickness of 3-10 μm. As a means of thinning, polishing or etching of a conventional silicon substrate from a back surface is considered, but the means is less controllable in uniformly reducing the thickness of an initial substrate, which is usually 500 μm, to 10 μm or less.
For example, Japanese Translation of PCT International Application No. 2008-514011 shows a manufacturing method of a solid-state image sensor of back surface irradiation using an SOI wafer including a single crystal silicon layer on a silicon oxide layer.
In the manufacturing method shown in Japanese Translation of PCT International Application No. 2008-514011, the SOI wafer including the silicon oxide layer formed on a base wafer, and the single crystal silicon layer formed on the silicon oxide layer is used. The method includes forming a photoelectric conversion element on the single crystal silicon layer with a light-receiving section facing the silicon oxide layer of the single crystal silicon layer, forming an interconnect layer on a surface of the single crystal silicon layer which is opposite to the surface closer to the silicon oxide layer, and selectively removing the base wafer under the silicon oxide layer.
As representative manufacturing methods of an SOI wafer, roughly two types of separation by implanted oxygen (SIMOX) and bonding are known. The SIMOX is a manufacturing method of an SOI wafer utilizing the feature that a silicon oxide layer is formed inside a silicon substrate and a recrystallized silicon layer is formed near the surface of the silicon substrate by ion-implanting highly concentrated oxygen into the silicon substrate with highly accelerated energy and performing heat treatment. This method accurately controls the depth for ion implantation, thereby providing excellent uniformity of the thickness of the recrystallized silicon layer formed near the surface of the silicon substrate. However, a non-single crystal silicon oxide layer occurs in the heat treatment, and the recrystallized silicon layer is formed on the non-single crystal silicon oxide layer, thereby causing a large number of crystal defects in the recrystallized silicon layer. Although considerable efforts have been made to reduce the crystal defects, the problem is not yet overcome, since the mechanism of occurrence of the defects is intrinsic. The defects are considered difficult to overcome in the future.
Then, wafer bonding was suggested. A Unibond technique (Smart Cut (registered trademark)) is practically the current mainstream.
In the Unibond technique, a silicon oxide film is formed on the surface of a silicon substrate, hydrogen ions are implanted via the formed silicon oxide film, and then the silicon oxide film of the silicon substrate is bonded to a base wafer (supporting substrate). After that, heat treatment is performed and the silicon substrate is separated in the implanted position, thereby providing a method (hydrogen ion implantation separation) of forming an SOI wafer. This technique is shown in, e.g., Japanese Patent No. 2959704, or Japanese Patent No. 3385972.
As the method of separating the hydrogen ions, a silicon oxide film is formed on at least one of two silicon wafers. Hydrogen ions or noble gas ions are implanted from above a first silicon wafer, thereby forming a defect layer (a sealing layer) inside the silicon wafer. Then, the silicon wafer is adhered to the other silicon wafer via the silicon oxide film and heat treatment (separation heat treatment) is performed to separate one of the wafers to be a thin film using the defect layer as a surface to be cleaved (surface to be separated). With further heat treatment (bonding heat treatment) is performed to strengthen the bonding, thereby forming an SOI wafer.